1. Field of the Invention
The invention relates to a method and structure for semiconductor devices, and more particularly to a method and structure for optical semiconductor devices.
2. Description of the Prior Art
Semiconductor devices are employed in a wide variety of electrical applications, for example, in central processors, memory devices, microwave devices, and light emitting devices.
One concern with the semiconductor devices is heat influencing on the semiconductor device. When temperature is raised by heat, reliability and life time of the semiconductor device are reduced. For optical semiconductor device, such as light emitting diode(LED), lateral laser, or planar laser, the efficiency of light emitting is also reduced by raising the temperature. An example of AlGaInP type LED, the light is emitted from electrons at an energy level of xcex93 bandgap combined with electronic holes. When the temperature is raised, partical electrons will jump on an energy level of X bandgap and then combine with the electronic holes, which may generate heat that reduces efficiency of internal quantum and efficiency of light-emitting.
The layers of many conventional light-emitting diodes (LEDs) are grown on an optically absorbing substrate having an energy gap less than the emission energy of the active region of the LED. The substrate absorbs some of the light generated within the active region, thereby reducing the efficiency of the device. An example of a prior art AlGaInhP LED of the double heterojunction type is shown in FIG. 1. An epitaxial layer 112 of n-doped (AlxGa1xe2x88x92x)0.5In0.5P, a light extraction layer 114 of (AlxGa1xe2x88x92x)0.5In0.5P and an epitaxial layer 116 of p-doped (AlxGa1xe2x88x92x)0.5In0.5P are grown on an n-type substrate 110 where xe2x80x9cxxe2x80x9d is a percent of chemical composition. A double heterojunction structure as a region of light emitting is formed between layers 112-116. An optically transparent window layer 118 of p-doped AlxGa1xe2x88x92x. As or GaP is grown on the epitaxial layer 116. The optically transparent window layer 118 enhances lateral conductibility of p-type region and further improves current spreading on the double heterojunction structure. On the other hand, the amount of xe2x80x9cxxe2x80x9d in the light extraction layer 114 determines wavelength of light emitting. The bandgaps of the epitaxial layers 112, 116, and the optically transparent window layer 118 are chosen so as to cause light to be generated in the light extraction layer 114 and to travel through the epitaxial layers 112, 116, and the optically transparent window layer 118 without being absorbed. However, absorption of light does occur at the GaAs substrate 110 which causes downwardly emitted or directed light to be absorbed and reduces light-emitting efficiency of the light emitting devices.
There are several techniques for resolving the light to be absorbed by the substrate. A first technique is to grow the light-emitting devices on a non-absorbing substrate. However, a problem with this technique is that acceptable lattice matching may be difficult to achieve, depending upon the lattice constant of the substrate similar to that of the LED epitaxial layers. A second technique is to grow a distributed Bragg reflector between the LED epitaxial layers and the substrate. An increase in efficiency is achieved, since the distributed Bragg reflector will reflect light that is emitted or internally reflected in the direction of the absorbing substrate. However, the improvement is limited because the distributed Bragg reflector only reflects light that is of near normal incidence. Light that differs from a normal incidence by a significant amount is not reflected and passes to the substrate, where it is absorbed.
A third technique is to grow the LED epitaxial layers on an absorbing substrate that is later removed. A transparent xe2x80x9csubstratexe2x80x9d is fabricated by growing a thick, optically transparent and electrically conductive epitaxial layers formed thereon. The absorbing substrate is then removed by methods of polishing, etching, or wafer lift off. The thin wafers are so thin and susceptible to fragile that a rather thick substrate is required. However, a xe2x80x9cthickxe2x80x9d transparent substrate requires a long growth time, limiting the manufacturing throughout of such LEDs. Moreover, the epitaxy growth spends much time and costs.
A concern with current distribution of LED is considered. Depicted in FIG. 2, an n-type substrate 132 is on an n-type ohmic contact 130 that contains a composition of Au/Ge. A light extraction 134 is on the n-type substrate 132, which is a structure of single or double heterojunction, or a structure of multiple-quantum well. A p-type transparent window layer 136 is grown on the light extraction 134. A bonding pad 138 of p-type ohmic contact generally contains a composition of Au/Be or Au/Zn. A light emitted from the light extraction layer 134 results from current travelling from the bonding pad 138 to the p-type transparent window layer 136. However, partial current may laterally travel between the p-type transparent window layer 136. A part of current may be ineffective because it cannot be achieved to exterior of grain when current travels upwardly and is blocked by the bonding pad 138. A technique for resolving the problem is to grow a current blocking right below the bonding pad and whereby the current cannot directly travel downwardly to the light extraction layer 134. For example, shown in FIG. 3, a current blocking 140 of n-type layer, whose conductivity is different from that of the transparent window layer 136, is utilized to achieve the effect of current blocking. There are two current methods for fabricating the current blocking 140. A two-step epitaxy method is to grow sequentially the current blocking layer and the light extraction layer 134 on the substrate 132. The current blocking layer is etched to form the current blocking 140 of n-type layer and placed again into an epitaxy chamber for sequentially growing the transparent window layer 136. However, the epitaxy chamber is susceptible to pollution that influences the properties of epitaxial layers. A second method is to utilize selectively local diffusion. However, it is difficult to control the fabrication condition.
It is an object of the present invention to provide a method for forming metallic substrate replacing conventional semiconductor substrate. The reliability and duration time of a semiconductor device may be enhanced by high thermal and electrical conductivity of the metallic substrate. Moreover, the metallic substrate also can enhance efficiency of light output for an optical semiconductor device.
It is another object of the present invention to provide a method for forming a mirrored or rough surface between the metallic substrate and the semiconductor layers for a light-emitting device. An acceptive mirrored surface may be a metallic surface or one caused by the differential of refractive index. The mirrored or rough surface can reorient downward lights from the light extraction layer and enable the downward lights to be far away from surface of grain so as to enhance the efficiency of light emitting.
It is yet an object of the present invention to provide a technique of metallic substrate. A current blocking layer below a light extraction layer is provided to block current and to enhance the efficiency of light emitting.
It is yet another object of the present invention to provide a method for forming a metallic layer as a temporary substrate whereby thin layers of semiconductor may be took out for other applications.
In the present invention, a method for forming a semiconductor device with a metallic substrate. The method comprises providing a semiconductor substrate. At least a semiconductor layer is formed on the semiconductor substrate. A metallic electrode layer is formed on the semiconductor layer. The metallic substrate is formed on the metallic electrode layer and the semiconductor substrate is removed. The metallic substrate has advantages of high thermal and electrical conductivity, that can improve the reliability and life-time of the semiconductor device.